In semiconductor applications using metrology systems, one often encounters substrates (e.g., semiconductor wafers) that are non-planar to such a degree that they cannot normally be processed or handled by the inspection system. Such wafers may be warped as a result of upstream manufacturing processes (e.g., metal deposition) that change the planar flatness and cause these wafers to be bowl-shaped, dome-shaped, or asymmetrical (twisted). Metrology tools, such as bright field tools, often require substantially flat wafers. Wafers are typically clamped with a chuck to flatten them for metrology or processing. Distorted wafers often cannot be clamped on a chuck to flatten them sufficiently for inspection in a metrology system. Consequently, a significant loss of yield through the metrology system can result from the warping of the wafers.
The type of chuck used to clamp the wafer depends on the nature of the processing. For example, electrostatic chucks are used in vacuum systems. Atmospheric systems can use vacuum chucks, which may be either passive or active in nature. Passive vacuum chucks typically have vacuum zones delineated by rings on the surface of the chuck connected by low conductance apertures. This type of chuck is only effective for domed wafers up to approximately 400 μm of warpage and is not effective at all for bowled or asymmetric wafers.
Prior art active methods for clamping wafers in atmosphere involve a similar segmented zone vacuum chuck design. However, independent solenoid valves control the vacuum applied to each zone. This allows the clamping method across the wafer to be timed and makes the same chuck useful for both 200-mm wafers and 300-mm wafers. Employing active vacuum zones on a chuck may yield some advantage for dished and asymmetric wafers. Unfortunately, the degree of effectiveness is generally not known a priori and, consequently, effective chucking of warped wafers often requires expensive and complicated software and hardware.
Thus, there is a need in the art, for a method for chucking warped substrates and an apparatus for implementing such a method.